In developed and developing countries, primary treatment and disinfection of waste water discharges from collection systems and waste water treatment facilities is the first step to improving water quality. Typically, secondary and tertiary waste water treatment processes are added to provide additional treatment of the primary effluent.
Primary treatment removes large and dense solids via screening and gravitational settling, allowing neutrally-buoyant matter to pass into the secondary treatment process or receiving body of water. Primary treatment utilizing gravitational settling or clarification is recognized as removing 20-33% of the organic load as measured in Biochemical Oxygen Demand (BOD). Secondary treatment removes another 50+% of the organic load by converting the BOD to biomass, in the form of bacteria, and CO2.
Secondary treatment provides an environment of adequate temperature, volume, mixing, and oxygen, or the absence of oxygen in anaerobic processes, to sustain the bacterial population necessary to consume the BOD and nutrients remaining in the waste water after primary treatment. New organic matter enters the treatment facility continuously so a portion of the existing bacterial population is removed from the process to promote the growth of new bacteria. The effectiveness of primary treatment directly affects secondary process or the receiving body of water if discharged from the collection system.
Primary clarifiers or settling basins are recognized as being the most economical means to reduce BOD as there is little energy required and no biomass to maintain. Primary treatment creates no biomass and therefore requires no aeration energy; no process controls to monitor the biomass to determine the health of the biomass; no separation or removal of bacteria by moving to a side-stream digester; no aeration of the digester; and no dewatering and disposal of surplus bacteria, also called secondary sludge. The lack of complexity of primary treatment is well suited for developing nations to promote recovery of surface waters and aquifers, resulting in a reduction in health issues.
Existing primary clarifiers may be circular or rectangular tanks and are volumetrically and geometrically sized to provide a horizontal fluid velocity lower than the solids settling velocity. The horizontal travel time and distance of the liquid from the inlet to the effluent weir or decanter must be greater than the settling time and distance of the suspended solids so that solids settle out prior to reaching the effluent weir or decanter. These settled solids contain a majority of the BOD in raw sewage. The effectiveness of this first stage is important because the more solids that exit the primary clarifier, the lower the BOD entering the secondary treatment process or the effluent-receiving body of water.
The '197 Application discloses an improved screen decanter with an ultrafine screen (also referred to herein as a screen box or “SBX”) in the form of a box, oval, or cylinder that is controllably driven in the vertical direction to optimize the exposure of the screen to varying wastewater levels and that can be lifted from the wastewater for backflushing and sterilization in a dedicated overhead apparatus. Because the motion of the screen assembly is only vertical, the required footprint in the tank can be relatively small. An air scour header provides air bubbles to air scour the screen surface. The application further discloses a low profile screen box useful for wastewater systems having high flows, limited surface area to place a screen box, and/or shallow active tank volumes of existing primary clarifiers, where multiple screen boxes or racks may be ganged in parallel to provide the necessary screen surface area at a controlled screen loading rate.
The '197 Application further discloses a deflector plate that increases the horizontal travel distance to the screen surface for solids that may be disturbed and start to move towards the screen.
A baffled lifting column and combined stub effluent drain pipe for an SBX are also disclosed in the '197 Application. The baffled lifting column is a slotted or perforated circular pipe that is connected to an effluent pipe or hose below the weir or dcanter. The lifting columns are centered in the SBX with openings to encourage flow distribution through the screen. A long rectangular screen rack has 3 lifting columns centered and equally spaced in the screen racks. Preferably, the open area of the baffled lifting column is lowest at the bottom and increases with elevation, creating head loss at the lower portion of the lifting column to equalize travel distance and pressure, and thus to equalize flow through the screen from the lowest point to the highest point of liquid contact.
An apparatus and method for simply and automatically preventing fouling of the upstream surface of any screen assembly is disclosed by the '247 Application.
In continued use of fine-screen apparatus in wastewater treatment, it is important to address potential fouling and blockage of the screening as a potential operational problem that can lead to inefficiency because of time lost to clean and/or replace clogged screens. Additional maintenance issues are typical in prior art operations, especially in high flow-volume situations such as municipal waste water treatment plants.
What is needed in the art is an improved screen decanter that maximizes decanter throughput by increasing flow uniformity and hence total flow through the screen element and that increases operational efficiency by increasing the time interval between required screen cleanings and/or screen replacements.
It is a principal object of the invention to maximize flow rate and volume of waste water effluent through a wastewater treatment system without fouling decanter screens prematurely or increasing the overall footprint of a screen box assembly, and thus without increasing the overall footprint of the primary treatment facility.
To enable this principle, it is a further object of the invention to control the waste water flow through a screen decanter so that all portions of all screens experience approximately the same flow rate, thus minimizing localized, high-peak flow regions that can clog portions of fine screens, and maximizing decanter throughput.